1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with a hydrostatic sealing assembly with improved hydraulic balance useful in sealing a shaft of a reactor coolant pump used in a nuclear power plant.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F. and 2500 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies--lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are located concentric to, and near the top end of, the pump shaft and their combined purpose is to provide for zero reactor coolant leakage along the pump shaft to the containment atmosphere during normal operating condition. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in U.S. Pat. Nos. to MacCrum (3,522,948), Singleton (3,529,838), Villasor (3,632,117), Andrews et al (3,720,222) and Boes (4,275,891) and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
The lower primary sealing assembly is the main seal of the pump. It is typically a hydrostatic, radially tapered "film-riding", controlled leakage seal whose primary components are an annular runner which rotates with the pump shaft and a non-rotating seal ring which is attached to the housing of the lower sealing assembly. Representative examples of such hydrostatic seals are the one disclosed hereinafter in thee introductory portion of the detailed description section of the subject application, and the ones disclosed in the third cross-referenced application and in the MacCrum, Singleton, Villasor and Andrews et al patents.
Historically, the pump shaft seals constitute the main problem area for the reactor coolant pumps and significantly contribute to the utilization factor in nuclear power plants. The seals must be capable of breaking down the high system pressure (about 2500 psi) safely. Whereas the tandem arrangement of three seals is used to break down the pressure, the lower main seal absorbs most of the pressure drop (approximately 2250 psi). Being a hydrostatic "film-riding" seal, the lower seal is designed to "lift off" (separate) at low system pressures without pump rotation. The lifting force is produced by a hydrostatic pressure force present in the gap between the stationary seal ring and the rotating runner. A closing or seating force, which must balance the lifting force, is produced by the system pressure acting on surfaces opposite the film surfaces of the seal ring and runner.
One of the potential problems associated with the lower seal stems from the preference to use a very pure grade of aluminum oxide as the faceplate material for the seal ring and runner. Use of such material is advantageous since it is harder than crud particles (usually iron oxide) which are small enough to enter the gap between the faces of the seal ring and runner, but large enough to lodge part way through the gap. However, a major disadvantage of aluminum oxide is that it is basically incompatible in rubbing against itself. If the seal faces momentarily contact while in motion, then usually some damage can be expected. If the rub is heavy enough, the faces are very seriously damaged and in some cases the thermal shock can lead to cracking and breakup of the structure.
For this reason, seal operating limitations and requirements are set very strictly. A minimum differential pressure (200 psi) must be maintained to establish a stable film between the faces and prevent rubbing when starting the pump. A minimum leakrate (0.2 gpm) is required for cooling and, most importantly, as a means of determining that an adequate film thickness exists prior to starting. Clearly the seal is most vulnerable at plant startup since the available differential pressure is low and crud may be present. Once the film is established, however, there is practically no seal face wear and the seal has a very long life expectancy.
Consequently, as long as a film riding seal continues to be preferred and faceplate materials such as aluminum oxide are employed, a need will remain for an effective way to prevent rubbing together of the faces of the seal ring and runner of the lower hydrostatic primary sealing assembly.